1. Field of the Invention
The present invention generally relates to methods for etching of molybdenum for fabricating a metal mask to be used in electronic packaging and semiconductor fabrication processes and, more particularly, to a method of etching molybdenum with an etchant which contains no cyanide and which does not emit noxious fumes when heated.
2. Description of the Prior Art
Current practice in molybdenum etching is to use ferricyanide. See, for example, A. F. Bogenschfitz, W. Braun, and J. L. Jostan, Metalloberflaiche, 29(9), 451-455 (1975), M. Beyer, A. F. Bogenschiitz, and J. L. Jostan, Metalloberflache, 29(10), 506-511 (1975), and J. D. David and M. T. Kurdziel, Metal Finishing, 86(5) 47-49 (1988). The ferricyanide mask etching operations for molybdenum is performed at pH.apprxeq.13: EQU 6Fe(CN).sub.6.sup.3- +Mo+8OH.sup.- .fwdarw.MoO.sub.4.sup.2- +4H.sub.2 O+6Fe(CN).sub.6.sup.4-
Ferrocyanide is then recycled with ozone: EQU 2Fe(CN).sub.6.sup.4- +O.sub.3 +H.sub.2 O.fwdarw.2Fe(CN).sub.6.sup.3- +2OH.sup.- +O.sub.2
Little etch waste results, but considerable quantities of rinse waste result, however, which are difficult to deal with because of the cyanide content of the iron complexes.
In addition, the etchant pH must be maintained around 13 to avoid polymerizing molybdate to molybdenum oxide and polymolybdate gels. This high pH, however, often results in resist delamination and resist stripping from the molybdenum. This limits the choice of photoresists to those that are not resistant to bases.
An etchant containing ferricyanide has been devised at neutral pH (.about.4) for molybdenum and tungsten etching is described in U.S. Pat. No. 4,995,942, but the cyanide bearing waste is still a disposal problem. Ferricyanide produces chemical wastes which are difficult and expensive to dispose because it contains cyano complexes.
Ferric nitrate is an alternative etchant which has been documented as an effective molybdenum etch. See D. M. Allen,
The Principles and Practice of Photochemical Machining and Photoetching, 81, Adam Hilger, Boston (1986). Ferric nitrate etches anisotropically with respect to grain boundaries of the crystal structure of the molybdenum material. The molybdenum that meets the thickness uniformity and flatness criteria necessary for metal evaporation and screening masks has a high degree of orientation in its grain structure. This results in diamond shaped holes, as shown in FIG. 1, when the molybdenum is etched with ferric nitrate, regardless of the shape of the artwork. This is unacceptable for molybdenum masks. In addition, during the etch process ferric nitrate produces nitric acid, which has a boiling point of 83.degree. C., making it very volatile in the processing environment.
The need thus exists for a molybdenum etchant, which contains no cyanide, even complexed cyanide, which yields good etch rates, etch profiles, and etch quality. The etchant and its waste must be easily treatable by conventional neutralization methods to yield nonhazardous effluent and sludge cheaply. The etchant also must not attack conventional polyphenolic photoresists.